Iron-silicon alloys and treatment thereof



United States Patent 3,223,602 IRON-SILICON ALLOYS AND TREATMENT THEREOFHans Werner Wawrousek, Pittsfield, Mass, assignor to genltzral ElectricCompany, a corporation of New or No Drawing. Filed May 17, 1961, Ser.No. 110,663 6 Claims. (Cl. 204-1405) The present invention relates toiron-silicon alloys, and more particularly to a process of treatingiron-silicon alloys of silicon content higher than about 3 /z% to renderthem suitable for use in electromagnetic apparatus and to facilitatetheir processing for this purpose.

In the construction of cores for electromagnetic apparatus, such astransformers, it has been customary to use laminations of silicon steelalloy having a silicon content of about 3%%. Silicon steel containingmore than that amount of silicon has not generally been used by theindustry because the addition of silicon to iron or steel is well knownto impart brittleness thereto, and increasingly so above 3% silicon, sothat even in very thin sheets, electrical steels having a siliconcontent above about 5% are extremely difficult to handle without risk ofbreakage during the assembly procedures. On the other hand, it is knownthat the higher the silicon content, the lower will be the hysteresisloss, and the lower the coeflicient of magnetostriction. The latterfactor provides an advantage in the use of higher silicon-content steelin that transformer noise due to magnetostriction is thereby greatlyreduced.

It is an object of the invention to provide silicon steel material ofthe above type which has good ductility coupled with low coetlicient ofmagnetostriction and low hysteresis loss.

It is a particular object of the invention to provide a method of simplyand economically improving the ductility of high silicon contentelectrical steel.

Other objects and advantages will become apparent from the followingdescription and the appended claims.

With the above objects in view, the present invention relates to amethod of making ductile, highly grain-oriented silicon steel containingabout 3.5% to 5% silicon which comprises alternately rolling and heattreating such silicon steel sheet material for reducing it to thedesired thickness and for developing the grain crystals therein in ahigh degree of directional orientation, and electrolytically polishingthe thus processed silicon steel material. The electrolytic polishingstep of the above-described process has been unexpectedly found toremarkably improve the ductility of the high silicon contentgrain-oriented silicon steel material.

In the usual processing of silicon steel strip to produce material ofgood magnetic and electrical properties, ingots of silicon-iron alloycontaining the desired amount of silicon are hot-worked usually byrolling to less than 0.15" strips or sheets called band. These are thensubjected to varying schedules of unidirectional cold rolling. The usualpractice comprises cold-rolling the material in one or more stages froma thickness of about 0.10" to about 0.014" with intervening heattreatments, followed by a decarburizing heat treatment and a final stagetexturedeveloping and purifying heat treatment at temperature in excessof 1100 C. in a hydrogen atmosphere. This latter heat treatment normallydevelops in the steel a crystal structure (texture) so oriented thatgood magnetic properties are obtained in the strip. Such crystalstructure is developed by secondary recrystallization, as known in theart, and as described in detail, for example, in US. Patent 2,986,485.The final heat treatment also normally removes impurities such as sulfurand carbon which may cause excessive watt loss in the oriented strip.The preferred crystal orientation resulting from the above treatmentwhich provides the desirable magnetic properties sought in the finallyprocessed electrical silicon steel is conventionally designated in theart as a (110) [001] crystallographic orientation, the notation being interms of Miller Indices.

When the silicon steel material contains more than about 4% silicon, andespecially above 5% silicon, it is so brittle that it is readily subjectto the risk of breakage during the subsequent handling of the processedsteel.

I have found, however, in accordance with the invention that siliconsteel containing up to about 5% silicon, after being processed in theusual way as described above, may be markedly improved in ductilitysimply by electrolytically polishing its surface. By this simpleexpedient, the quite brittle, fully processed silicon sheet is endowedwith a bend ductility such as characterizes a 3.25% silicon steel alloy.

A particular electropolishing procedure which has been foundsatisfactory to obtain improved ductility comprises treating the siliconsteel material in an electrolytic bath composed of a solution ofphosphoric acid and a suitable oxidizing agent such as chromium trioxide(CrO In an experiment carried out in practicing the invention, Epsteinstrips of electrical steel alloy containing 3.8% silicon and fullyprocessed to obtain optimum magnetic properties as above described weremade the anode in an electrolyte bath composed of a solution of 100grams chromium trioxide in 5 30 cc. of phosphoric acid, the cathodebeing a stainless steel member. The bath was at a temperature of C. andan electric current density of 2-3 amps/in. at a voltage of about 20volts was applied for 1015 minutes. Following such treatment whichresulted in a mirror surface on the samples, it was found that theEpstein strips had excellent ductility and could be bent into virtuallyany shape and at extreme angles without breaking. In contrast, Epsteinstrips identical to those described except that they had not beensubjected to the electrolytic polishing treatment were found to fracturereadily when bent through an angle of 180.

In the above electropolishing treatment, the bath temperature may be inthe range of 70 to C., the current density in the range of 1 to 3amps/in. and the time from about 10 to 20 minutes, while providingsatisfactory results.

In another series of experiments, the same treatment was applied tosimilar, fully processed Epstein samples containing 5.2% silicon. Priorto the electropolishing step, these samples could not be bent at allwithout breaking. After the electropolishing treatment which produced asatin-like finish on their surfaces, samples could be bent up to withoutfracturing. Other experiments have shown that abrading the fullyprocessed high silicon content alloy will not produce the improvedductility afforded by the present invention, and also show that treatingthe silicon steel material with pickling solutions and procedures knownin the art likewise are not effective for this purpose.

The reason why electrolytic polishing in accordance with the inventionprovides such marked improvement in ductility in high silicon contentsteel alloys is not fully known. In part, such improvement may beattributable to removal by the polishing step of an oxide layer formedby the final annealing process which results in trapping hydrogen in thesteel. However, mere removal of such a layer is apparently notsufficient in view of the demonstrated ineffectiveness of theaforementioned pickling or abrading steps. It appears possible that thenascent oxygen observed to be evolved at the anode reacts with thesilicon steel material, and the elevated temperature generated by thisand other reactions occurring at the anode results in liberating thehydrogen from the silicon steel and reduces the embrittling which it maycause. It should be understood, however, that I do not intend to bebound by the explanation thus presented, since other theories may verywell apply to explain the results obtained. For example, it isconceivable that silicon steel material with over 3.5% silicon may benotch-sensitive, that is, readily susceptible to fracture by bendingbecause of the presence on its surface of notches, scratches, or thelike however small they may be, and the improved ductility impartedthereto might well result from the complete removal or smoothing of suchsurface irregularities by the electropolishing treatment describedherein.

A typical procedure which may be employed in processing the siliconsteel alloy prior to subjecting it to electropolishing in accordancewith the invention is as follows, it being understood that the specificprocedure is given by way of example only:

Raw iron scrap containing the usual proportion of impurities such asphosphorus, sulfur, chromium, nickel, aluminum, manganese, and copper ismelted in a furnace with about 3.56% by weight of silicon (preferablyabout added thereto. With the temperature of the melt adjusted to about1600 C., the mixture is poured into an ingot mold. After solidification,the ingot is hot rolled to a strip 100 mils thick, and then rolled to anintermediate gage, e.g., 30 mils. The strip is then heat treated in anopen anneal, cooled and cold rolled to the desired final gage, e.g. 14mils. The strip is then decarburized by heat treatment in a wetnon-carburizing atmosphere at 800 C. Thereafter, the thus-treated stripsare subjected to an anneal at about 1175 C. for about eight hours or asufficient period to grow secondary crystals of optimum size and properorientation and to further purify the strip by the removal of residuesof carbon, sulfur, oxygen, and other impurities.

Following the above treatment, the thus fully processed steel issubjected to the electrolytic polishing treatment to improve itsductility in accordance with the invention, as described above.

There is thus provided in accordance with the invention, a simple,economical, and convenient process whereby it is possible to markedlyimprove the ductility of high silicon content electrical steel sheetmaterial while obtaining the low hysteresis loss and the lowmagnetostriction which normally characterize such silicon steelmaterial. In this way, substantial reduction in transformer noise can beaccomplished, without sacrifice in good electrical properties.

The expressions sheet material and sheet as used in the appended claimsare intended to include such forms as sheets, strips, tapes, and otherlaminar shapes.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comewithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of making ductile highly grain oriented silicon steelcontaining from above about 4% to about 6% silicon which comprisesalternately rolling and heat treating silicon steel sheet materialcontaining from above about 4% to about 6% silicon for reducing the sameto the desired thickness and developing the grain crystals therein in ahigh degree of directional orientation, and electrolytically polishingthe thus-processed silicon steel material.

2. The method of making ductile highly grain oriented silicon steelcontaining from above about 4% to about 6% silicon which comprisesalternately rolling and heat treating silicon steel sheet materialcontaining from above about 4% to about 6% silicon for reducing the sameto the desired thickness and developing the grain crystals therein in ahigh degree of directional orientation, and electrolytically polishingthe thus-processed silicon steel material by making the silicon steelmaterial the anode in an electrolytic bath containing phosphoric acid.

3. The method of making ductile highly grain oriented silicon steelcontaining from above about 4% to about 6% silicon which comprisesalternately rolling and heat treating silicon steel sheet materialcontaining from above about 4% to about 6% silicon for reducing the sameto the desired thickness and developing the grain crystals therein in ahigh degree of directional orientation, and electrolytically polishingthe thus-processed silicon steel material by making the silicon steelmaterial the anode in an electrolytic bath containing phosphoric acidand an oxidizing agent and heated to a temperature of about to C.

4. The method as defined in claim 3, wherein said oxidizing agentcomprises chromium trioxide.

5. The method of making ductile highly grain oriented silicon steelcontaining from above about 4% to about 6% silicon which comprisesalternately rolling and heat treating silicon steel sheet materialcontaining from above about 4% to about 6% silicon for reducing the sameto the desired thickness and developing the grain crystals therein in ahigh degree of directional orientation, and electrolytically polishingthe thus-processed silicon material by making the silicon steel materialthe anode'in an electrolytic bath containing phosphoric acid and anoxidizing agent and heated to a temperature of about 70 to 100 C., andapplying a current to the bath sufiicient to produce a current densityof 1 to 3 amperes/in. for a period of 10 to 20 minutes.

6. Electrical silicon steel having good ductility coupled with lowcoefficient of magnetostriction and low hysteresis loss comprisingsilicon-iron alloy sheet material containing from above about 4% toabout 6% silicon having a high degree of grain orientation and made bythe process as defined in claim 1.

References Cited by the Examiner UNITED STATES PATENTS 2,334,699 11/1943Faust 204-140.5

2,347,040 4/ 1944 Faust 204140.5

2,366,712 1/1945 Faust 204140.5

2,424,674 7/1947 White 204140.5

2,986,485 5/1961 Fitz 148112 OTHER REFERENCES Jacquet, P.: MetalFinishing, February 1950, pages 55-60.

JOHN H. MACK, Primary Examiner.

1. THE METHOD OF MAKING DUCTILE HIGHLY GRAIN ORIENTED SILICON STEELCONTAINING FROM ABOVE ABOUT 4% TO ABOUT 6% SILICON WHICH COMPRISESALTERNATELY ROLLING AND HEAT TREATING SILICON STEEL SHEET MATERIALCONTAINING FROM ABOVE ABOUT 4% TO ABOUT 6% SILICON FOR REDUCING THE SAMETO THE DESIRED THICKNESS AND DEVELOPING THE GRAIN CRYSTALS THEREIN IN AHIGH DEGREE OF DIRECTIONAL ORIENTATION, AND ELECTROLYTICALLY POLISHINGTHE THUS-PROCESSED SILICON STEEL MATERIAL.